Intensity-modulated radiation-therapy (IMRT) is a state-of-the-art technique for administering radiation to cancer patients. The goal of a treatment is to deliver a prescribed amount of radiation to the tumor, while limiting the amount absorbed by the surrounding healthy tissues and organs at risk.
The primary delivery tool for IMRT is a linear accelerator that rotates on a gantry around the patient, emitting “modulated” beams of X-rays. This modulation is accomplished by means of a device known as a multi-leaf collimator (MLC) which is attached to the accelerator. The MLC consists of a number of separate elements, the leaves, that can independently move and are placed side-by-side to shape the radiation beam. These adjustable heavy-metal leaves act as a filter, blocking or allowing radiation through in a precise manner controlled by a computer, in order to tailor the beam shape to the shape of the tumor volume while minimizing exposure of the neighboring structures.
Treatment proceeds by rotating the accelerator around the patient and coordinating the leaf movements in the MLC so that the radiation delivered conforms to some desirable dose distribution at each gantry (beam) angle. In addition to knowing the beam angles, one must also know the MLC aperture for all gantry angles, that determine the beam fluence (or intensity) at each point from every gantry angle. These intensity profiles are represented by fluence maps.
Planning an IMRT treatment requires calculation of the doses from radiation beams, which can be determined through fluence maps (each consisting of different beamlet intensities) or directly from the characteristics of the treatment plan (one of the most important being the MLC positions). The longer an MLC leaf is open at a certain position, the higher the fluence at that position and the higher the dose delivered to the tissue along a straight path from that position (plus some surrounding tissue).
Adequate modeling of a multi-leaf collimator (MLC) is essential for accurate dose calculations in intensity-modulated radiation-therapy (IMRT) treatments involving dynamic MLCs. For this reason, modern treatment planning systems incorporate MLC characteristics such as the leaf end curvature, MLC transmission, and the tongue-and-groove effect.
Many MLC devices use leaves with rounded tip ends, which produces increased transmission in the region near the leaf tip (where the leaf height is smaller) and some radiation passes even through completely closed leaves. This is typically modeled considering leaf edges as straight and accounting for the transmission through rounded leaf ends by computing the fluence after shifting the leaf positions a certain leaf offset. Thus, leaves are pulled back so that the gap for all leaf pairs is increased by twice this leaf offset value, which is defined as the dosimetric leaf gap (DLG) parameter. Hence, the fluence for a completely closed pair of leaves is computed as the fluence produced by a gap equal to the DLG parameter.
Transmission through the MLC is defined as a ratio between the doses from an open field and a field with a fully closed MLC. Transmission between leaves (interleaf transmission) is higher than the average transmission due to the thin layer of air between leaves, which reduces the ability of the MLC to shield the beam. Therefore, many MLC models have a “tongue-and-groove” design, where the sides of adjacent leaves interlock in order to minimize interleaf transmission. The tongue-and-groove design is illustrated in FIGS. 1 and 2.
This design reduces interleaf transmission, but it increases the effective leaf width when leaves project out into the beam due to the protruding part of the leaves and, in general, whenever the leaf sides are exposed. The tongue-and-groove effect is modeled by modifying the fluence used to calculate the dose distribution. To this aim the projections of the leaf sides are typically extended a constant width in the direction perpendicular to the leaf motion, which produces a reduction in the resulting fluence map. However, this arrangement can produce underdosage between adjacent leaf pairs in asynchronous MLC movements due to this region being further shielded by the tongue of opposing leaf sides in different phases of treatment delivery. This underdosage is known as the tongue-and-groove effect (TG effect). The tongue-and-groove effect is taken into account both in the actual fluence calculation for MLC apertures (static fields and arc fields) and for IMRT fields. The effect is more significant in IMRT treatments than in static MLC delivery techniques.
An exposed tongue in a field modifies the delivered fluence by blocking some additional radiation. The amount of blocking is proportional to the tongue width w. The tongue-and-groove is modeled in the treatment planning system by extending the leaf projections in the direction perpendicular to the leaf motion with a constant extension parameter. This parameter depends on the MLC model and is user-configurable in some treatment planning systems, while in others is fixed and cannot be modified in the treatment planning system configuration. As a consequence of these effects, the fluence map in the direction of leaf motion is increased by the dosimetric leaf gap (each leaf tip being pulled back DLG/2), while in the perpendicular direction it is reduced by a certain width at each leaf side. This difference between the nominal leaf edges and the fluence map used for dose calculations is shown in FIG. 3.
In general, IMRT plans may involve many highly irregular and small MLC apertures and in volumetric-modulated arc therapy (VMAT) individual leaves may repeatedly extend into the radiation field, giving rise to considerable TG effects. Proper modeling of all MLC characteristics is particularly relevant, therefore, in VMAT treatments. Nevertheless, it is difficult for a treatment planning system to fully consider the effects of the beam delivery system. Some investigators have reported that treatment planning system calculations are able to reproduce patterns of dose dips and peaks for a static test field with maximum TG effect, but it has been recently shown that the modeling of the tongue-and-groove modeling in treatments with dynamic MLCs is not accurate enough, especially for high resolution MLCs and for small MLC gaps.